Nowadays, it is commonly seen that backlight modules are used in electronic devices with planar displays, which includes devices as small as hand-held palm pilots and as large as big-screen TVs. The design challenge of a backlight module is to generate uniform illumination across the LCD surface and luminance that is high enough to produce good contrast in a day environment, so that most backlight modules had configured therein a brightness enhancement film (BEF) or a dual brightness enhancement film (DBEF) for such light management. It is noted that most optical film used in backlight module, such as a BEF or a DBEF, is thin matte surface reflective polarizer with tiny triangular structures on the film surface, as that shown in FIG. 1A. Conventionally, for enabling an optical film to guide an incident light thereof to its frontal and normalized view angle of the backlight module while enhancing the light emission efficiency of the same, the optical film can be a film fabricated from at least two overlapping BEFs or DBEFs for enhancing collecting light ability. Please refer to FIG. 1B, which is a schematic diagram showing light paths of beams traveling in a conventional optical film. In FIG. 1B, only a potion of the optical field generated from the light guide plate 11 and guided toward the optical film 10, represented by the light 91, is going to travel pass the optical film 10 by a direction normal to the light-emitting surface of the optical film 10 while the rest of the optical field, represented by the light 91, will be guided back to the light guide plate 11, by that the light emission efficiency of the optical film 10 is adversely affected as the light is directed to travel back and forth in the optical film 10.
In U.S. Pat. No. 4,791,540, entitled “Light fixture providing normalized output”, an optical structure is provided which is substantially a light guide plate having two optical films placed thereon while each optical film is configured with microstructures on a surface thereof. The aforesaid optical structure is capable of enabling light to be reflected back and forth between the two films and the light guide plate so as to provide normalized output. Instead of modifying the optical field of the light guide plate itself, the optical structure will reflect to optical field back to the light guide plate where it is redistributed and scattered so as to be redirected to the optical film for emitting. As the microstructure formed on the optical film is going to refract those incident beams of certain specific incident angles for enabling the same to be emitted away from the normal of the optical film while enabling others to be refracted/reflected back to the light guide plate where they are redistributed and scattered so as to be redirected to the optical film in an back and forth manner until they are able to be emitted away from the normal of the optical film. Thereby, eventually all the beams from the light guide plate will be emitted away from the normal of the optical film so that the luminous efficiency is enhanced. However, also because of the light is reflected back and forth between the optical films and the light guide plate, the energy of the light is lost during the process by a great amount.
In U.S. Pat. No. 5,863,113, entitled “Plane light source unit”, an optical film with microstructure being adapted for backlight module is provided, in which the plural prism units formed on the optical film on a surface thereof facing toward a light guide plate will reflect/refract the beams emitted from the light guide plate for enabling the same to be emitted away from the normal of the optical film. The aforesaid optical film is able to provide a normalized output directly without having the beams to be reflected back and forth between the optical film and the light guide plate. However, the friction between the prism units of the optical film and the light guide plate is going to cause damage to the prism units, defecting the backlight module accordingly.